Loop input system for radio receivers



Aug. 21, 1945. G, l... BEERS 2,383,236

` LOOP INPUT SYSTEM FOR RADIO RECEIVES Filed Jan. 28,` 1943 `2 Sheets-Sheet 1 Aug. 21, 1945. G. L.. BEERS LOOP INPUT SYSTEM FOR RADIO RECEIVERS /Qfh&

Goyo` to a loop input system utilizing y input circuit for receivers.

,i using permeability tuning.

Patented Aug. 21, 1945 l `.2,383,286 LooPiNrU'r SYSTEM FOR RADIO 1 `ItEoElvERs George L. Beers, Haddonfeld, N. J., assigner to Radio VCorporation of America, a corporation i of Delaware l Application January 2s, 1943,seria1N0. 473,806

The `present invention relates to a loop input system for radio receiversand more particularly `a permeability tuned auto-transformer. 1

4'I'heuseWof permeability tuning in the design of broadcast and other receivers has become widespread in recent years due to low cost,compact ness, freedom frein `acoustic feed-back howl and other considerations, such as the lack of suitable materials for themanufacture of variablecapac-` itors. Although this method of tuning is best adapted for use with a capacity antenna device, such asa rod, or a capacity plate, capacity antenna` devices are not commercially acceptable due to body capacity effects, the appearance of n the rod extending beyond the receiver, and to the `extreme insensitivity in shielded buildings. It is therefore ldesirable to be able `to use a loop antenna in conjunction with a permeability tuned Withthe advent of permeability tuning systems there arose theneedxfor an ellicient loop circuit that `would be adapted for permeability tuning. Several loop antenna circuits employing permeability tuning `have been-devised butthey were found to be inefficient in thatthey have a considerablylower `sensitivity than a capacitor-tuned high quality loop. In one instance, there has been used a series variable inductance to tune the loop antenna and` aseries capacitor which serves `to tune the loop below `the band to be received; n l i 'Ihe average sensitivity of prior art serially connected permeability tuned loop antenna `circuits is ogt'he order of 10% of that of a variable capacitor tuned loop input circuit. Theimproved circuit according to the presentinvention has a sensitivity of approximately,40% of that obtaini able `from the` same loop antenna, `capacitively tuned. i

According lto the` invention the loop input system involves the use of a loop antenna which is `suitably tapped onto a variable permeability tuned inductance inthe input circuit ofarreceiver to provide a substantially uniform response across the broadcastor other tuning band and `to secure an appreciable increase lin sensitivity for a permeability tuned loop circuit over that which is possible with the usual series tuned loop `circuit It is therefore one of the objects of `the invention to provide an efficient means of using a loop antenna in conjunction a permeability tuning system.,

` Another objec't 'is .to provide efficient `coupling 14 claims. (o1. 25o-20) means .between the loop antenna and the permeability tuning system `of a receiver `whichlwill result in a substantially uniform response over the 1 broadcast or other tuning band;

A further objectV of` the invention is to secure l an appreciable increase insensitivity for` a permeability tuned loop circuit over that which is possible with tuned loop circuits known inthe art. 9 1 1 The invention will be further understood from the following description, when considered `in connection `with the accompanying drawings and its scope is pointed out in the appended claims.

l In the drawings, n l

Fig. l shows the basic circuitinvolving the invention; i n

^ Fig. 2 shows the equivalent n T couplingkarrangement used in Fig. l; and n Figs. 3, 4 and 5 are curves which willserveto explain the invention.

Referring now to Fig. 1, there isrepresented at]` I `a loop antenna havingja suitable number "of turns, with one end ofthe loop directly connected to ground while the other `or high potential end is tapped onto a permeability tuned 1 inducta'nce l coil 2. As will be shown hereinafter the tapping pointispreferably between 25 to 35% ofthe coil turnsfrocm the ground end. The coil 2 `and `a l shunt capacitor `3 constitutethe inputcircuit for coupling to the signal grid of atube 4 which may be either the. rst radio frequency amplifier `or the detector-oscillator tube `of a conventional receiver. AA powdered iron-core'movably 'mounted within and enteringv at the ground end of lcoil 2 serves to tune theinput `circuit to the signal frequency `desired to be received.

The manner inwhich the permeability tuned l circuit ofthe invention operates is vas follows:

`AS the'core 5 is `caused tomove into theLi end of the coiland progressively through the tentire coil 2 to tune the circuit-,throughout the `desired tuning range, the relative values `of the inductances L1 and Lzfand the mutual inductance M ytherebetween are continuously varied in such a Way to maintain a substantially `uniform `transfer of voltage from the loop to the grid of tubed. In

other words, as the input circuit is tuned throughoutthe frequency range,"the point to `which the loop is 'tapped isin effect simultaneously varied, the tapping point in eifect advancing progressively to a point above the actualtap during the initial adjustment `of the core `(high frequency end of the tuning range) and then recedingprogressively until it coincides with the actual tap when the core is adjusted toits fully entered `position with the coil (low frequency end of the tuning range). By reason thereof the coupling between the loop and input circuit is eifectively Varied throughout the tuning range to effect a unifo-rm transfer of energy.

In one particular embodiment satisfactory results have been obtained with an inductance coil of 360 turns, tapped at 90 turns or 25 percent, and with a loop of approximately 14 turns spaced over an axial length of 6 inches on a frame, ap-

proximately 2.0 by 36 inches for tting inside of a console cabinet.

In Fig. 2 the circuit of Fig. 1 is shown with the equivalent T network for the permeability tuned auto-transformer. The distributed capacitance of the loop'and the tapped portion L1 is denoted by C, shown dotted. The elements of the primary circuit are isolated to the left of dash line af-a. For the loop antenna of inductance L1 and the auto-transformer utilized in a particular embodiment the maximum equivalent inductance o'f. (Lz-M) and (Li-i-M) in parallel was calculated and found to abe 963,171.. Calculation also shows that the primary resonant frequency for the above value of the equivalent inductance was 3.01 mc., a value considerably above the range to be received. For any other position of the core a higher value for the primary resonant frequency will result.

In Figs. 3 to 5 are shown a number of sensitivity curves obtained with coils of different inductance values and with the loop connected to varying tapping points. The ordinate of each curve sheet represents, in percent, the sensitivity of the present permeability tuned loop input system as compared with that of the same loop antenna, capacitively tuned. Forthe permeability tuned' coil 2 in Fig. 1, there were used coils of different inductance, one of which will be identied as A and the other as B. Coil A comprised 260 turns and coil B comprised 360 turns, and -both coils were universally wound to the same length (1%) on the same diameter coil form. The tuning core was the same in each case. The inductance of coil B varied from 196.5 to 2043/ih. and its Q varied from approximately '70 t0' 100 across the band. The inductance of coil A, approximately one half of coil B, varied from 94.9 to 990ML, and its Q Varied from 85 to 105 over the band.

Fig. 3 shows the effect of changing the L/C ratio of the circuit, coil A having approximately one-half the inductance of coil B. Curves I land 2 are both for a large (i. e., area=0.334 m.2) 14 turn loop connected to a 25% of the turns tap on coils A and B, respectively. Curves 3 and i are for the same loop connected to the 50% of `the turns tap on co-ils A and B, respectively.,

Thus, a 2 to 1 decrease in L/ C ratio causes only a small change in tuning range and a decrease in sensitivity at the high end of the band. An explanation of the effect of the two different tapping points will be deferred until the curves of Fig. 5 are explained.

The effect of increasing the loop turns from 14 to 20 for the case of coil B tapped at the 25% point is shown in Fig. 4. Curve 2 is the same as curve 2 in Fig. 3 for the 14 turn loop, and curve 5 is for the 20 turn loop. A slight increase in tuning range and' a small decrease in sensitivity is seen to result from the larger number of turns.

The curves of Fig 5 show two important effects due to connecting the loop to various tapping points. First, the tapping point on coil A was varied from 61/4 to 50 percent. A tap below 25% results in poor sensitivity. 'Ihe 25% tap permits a satisfactory tuning range, and at the same time results in practically optimum sensitivity at the high frequency end of the band. The 50%y tap results in a greatly reduced tuning range and in lower sensitivity at the high frequency end of the band. Second, the importance of inserting made without departing from the spirit and scope of the invention, as will be understood by those skilled in the art.

What I claim is:

l. In a radio receiver, an input circuit for said receiver having a. tuning inductor which is connected to ground at one end, a magnetic core movable in said inductor for tuning the circuit over a. frequency range, and a loop antenna. connected between a tap on said inductor and ground.

2. In a radio receiver, an input circuit for said receiver having a tuning inductor which is connected to ground at one end, .a magnetic Core movable in said inductor for tuning the circuit over a frequency range, and a loop antenna having one end connected directly to ground and the other end connected toa point on the inductor which is between 20 and 35 percent of the turns from the ground end of the inductor.

3. In a radio receiver, an input circuit for said receiver having a tuning inductor and a` magnetic `core movable in said inductor for tuning the circuit over a frequency range, 'and a loop antenna, connected between the ground end of said inductor and a tap on the inductor `which is between 20 `and 35 percent of the inductor turns from the ground end, said magnetic core entering the ground end of the inductor.

4. In a radio receiver, in combination, a tuning inductor adapted to'function as an autotransformer, a loopantenna connected in shunt to the primary of said auto-transformer and resonating therewith above the frequency range of the receiver, an input circuit including the `secondary of the auto-transformer, and a magnetic core movalblein the tuning inductor for tuning the input circuit.

5. The combination defined in claim 4 wherein the primary contains from between 25 to 35 percent of the number of turns ofl the tuning inductor. y

6. The combination defined in' claim 4 wherein the primary contains from between 25 to 35 percent of the number of turns of the tuning` inductor and the magnetic core enters the low potential end of the tuning coil. Y

7. In a radio receiver, an input circuit including an auto-transformer, al loop antenna connected inshunt across the primary of said transformer, and means for tuning the input circuit throughout its operating range` and for varying simultaneously therewith theinductance of the primary and secondary of said transformer ,and the mutual therebetweenso that a uniform transfer of energy is obtained between, the loop` antenna and the input circuit. l

8. In a radio receiver, an input circuit including an auto-transformer, a loop antennaonnected in shunt across theprimary of `said transformer, and means for tuning the input circuit i. assenso throughout its opratingrange and for Varying simultaneously therewith the ratio of the `primary to secondary inductance so that a uni-` transformer `and the mutual therebetween so` that a uniform transfer of energy is obtained between the loop antenna and the input circuit.

10. In a radio receiver, an electron discharge turbe, an input circuit including an inductance coil having one end grounded and its other end connected to the. signal control .grid of said tube,

a loop antenna having one end grounded and its other end connected to a tap on said inductance, and means for tuning the input circuit throughout its operating range and for simu1ta` i neously effecting a` progressive variationv in"`cou pling ibetween the loop antenna and the `input circuit.

11. In a radio receiver, an electron discharge tube, an input circuit including an inductance coil having one end grounded and its other end connected to the signal control grid ofisaid tube,

in coupling between the loop antenna and the input circuit to effect thereby a uniform energy` transfer from the loop to the signal grid of the tube.

. 12. An antenna input system for` radio receivers comprising a high-frequency auto-transformer, a ferromagnetic core movable relatively` thereto, a capacitance effectively in shunt with the secondary of said auto-transformer `and forming a secondary resonant circuit tunable by movement of said core to any desired signal `Within a range of frequencies,` an inductive element exposed1to `said signals and effectively in shunt lacross `the l primary of said auto-transformer forming a primary resonant circuit which is tuned by motion of said core to a frequency higher than the frequency of said` signal.

13. An antenna input system forradio receivers comprising Aa high-frequency coil having. one

end connected'to ground and a tapl at` a point i closer to said coil end,` a ferromagnetic core movable relatively thereto, -a capacitance effectively in shunt with the Whole `of said coil and` forming a secondary resonant circuit tunable by movement of said core to any desired signal within a range of frequencies, an inductive element eX- posed to `said signals and connected between the l coil tap and ground and forming with said coil a loop antenna having one end groundedf its other end connected to a` tap on said inductance, and a movable magnetic core cooperating with said inductance `coil for tuning the input circuit throughout its operating range and for simultaneously causing aprogressive variation portion a primary resonant circuit which is tuned by motion of said core to a frequency higher than-thefrequency of said signal;

14. An antenna, input system as defined in` claim 13 wherein the tap is at a point from bey tween 25 to 35% of the coil turns from the ground o-f the coil.

end, and the core is entered at the ground end GEORGE L. BEERS; 

